To optimize the thermal efficiency of large scale fossil fuel burning heat exchangers or boilers, it is necessary to periodically remove deposits such as soot, slag and fly ash from their interior heat exchanging surfaces. Typically, a number of cleaning device such as those known as sootblowers, are mounted to the exterior of the boiler. One type of sootblower has a lance tube which is inserted periodically into the boiler through a cleaning port located in the boiler wall. Positioned on the forward end of the lance are one or more cleaning nozzles. The nozzle discharges a pressurized cleaning medium, such as air, water steam or other solutions. The high pressure cleaning medium contacts deposits of soot, slag and fly ash and causes them to be dislodged from the internal structures of the boiler.
Conventional wall box assemblies serve a number of purposes. One purpose being to provide a support structure for the previously mentioned cleaning lances. Without a sealing wall box, during cleaning, combustion by-products would escape to the exterior of the boiler or air could enter the boiler through the gap between the cleaning lance and the cleaning port. Controlling leakage through the boiler access ports poses a number of significant design challenges. There is a requirement of sealing the opening to prevent boiler gases from leaking outside the boiler. Conversely, in many applications of negative pressure operating boilers, there is a desire not to admit fresh air in an uncontrolled manner through sootblower wall ports. Oxy-fuel boilers use a mixture of flue gas and oxygen as an oxidant instead of air, and therefore the uncontrolled introduction of air is undesirable. The wall box for a lance port must also provide a good seal against the lance tube during its operation for the reasons mentioned previously.
Some existing wall box assembly designs incorporate two pressurized air flow circuits and include a sealing air chamber and an aspirating air chamber. Both chambers are supplied with pressurized air and provide air to the wall box at a pressure greater than the internal operating pressure of the boiler. When the sootblower lance is inserted through the wall box for cleaning, positive pressure sealing air is provided to the wall box assembly. Once the cleaning lance is fully retracted, aspirating air is directed toward the interior of the heat exchanger through an annular array of ports. The orientation of the aspirating ports, along with the increased pressure of the aspirating air, restricts the outflow of combustion by-products from the cleaning port during normal operation of the boiler. Mechanical closure devices may be used to plug the wall ports between operating cycles of the sootblower.
The requirement of a wall box to have sealing airflow imposes efficiency limitations by requiring a constant source of compressed air. Purge airflow also imposes cost due to the equipment and plumbing required and gives rise to a source of system failure. It is desirable to reduce the reliance on sealing air while meeting acceptable sealing performance requirements for the wall box.